One of the earliest parts of brain myelination is called central tegmental tract, which is an extrapyramidal tract between the red nucleus (superiorly) and the inferior olivary nucleus (inferiorly). 

These tracts are seen as T2/FLAIR/DWI high signal intensity, and can be seen in multiple conditions; most importantly in epilepsy and antiepileptic drugs (vigabatrin), but also can be seen in hypoxic-ischemic encephalopathy, cerebral palsy, metabolic disorders and others ^1.